This application claims priority to Japanese Patent Application Number JP2001-248874 filed Aug. 20, 2001 which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of producing a magnetic recording medium and a magnetic recording medium, more particularly relates to a method of producing a magnetic recording medium of a metal magnetic thin film type including columnar structure magnetic particles and non-magnetic particles in a magnetic layer.
2. Description of the Related Art
In magnetic recording systems such as video tape recorders, for the purpose of improving picture quality, a greater increase in recording density has been strongly demanded. As a magnetic recording medium increased in recording density, a so-called xe2x80x9cmetal magnetic thin film type magnetic recording mediumxe2x80x9d wherein a ferromagnetic material composed of a metal or alloy is deposited directly on a non-magnetic base film to form a magnetic layer has been proposed and is attracting attention.
Here, as an alloy of a ferromagnetic material, a Coxe2x80x94Ni alloy, a Coxe2x80x94Cr alloy, a Coxe2x80x94O alloy, or other alloy can be mentioned. A film of the ferromagnetic material is formed on the non-magnetic base film by plating or a vacuum thin film deposition method. As a vacuum thin film deposition method, vacuum evaporation, sputtering, ion plating, etc. can be mentioned. As the non-magnetic base film, a polyester film, polyamide or polyimide film, etc. can be mentioned.
A metal magnetic thin film type magnetic recording medium is superior in coercive force, residual magnetization, squareness ratio, etc. and also in electromagnetic conversion characteristics at a short wavelength compared with a coating type magnetic recording medium having a magnetic layer formed by coating a magnetic coating material. Also, since the ferromagnetic material is deposited directly on the film, the magnetic layer can be made extraordinarily thin. Due to this, the demagnetization of recording and output loss due to thickness during reproduction can be reduced significantly so that good electromagnetic conversion characteristics can be obtained.
Further, there is no need to mix a non-magnetic binder and additives in the magnetic layer, so the filling density of the magnetic material is increased and the magnetic flux density can be increased. In this way, a metal magnetic thin film type magnetic recording medium has many advantages.
To further improve the electromagnetic conversion characteristics of a thin film type magnetic recording medium and obtain a higher output, so-called xe2x80x9cvacuum oblique evaporationxe2x80x9d for depositing a ferromagnetic metal material by making it obliquely strike the non-magnetic base film has been proposed.
A metal evaporated type magnetic tape (metal evaporated tape) having a magnetic layer formed by vacuum evaporation has a high manufacturing efficiency and stable characteristics. Due to this, metal evaporated tapes have already been put into practical use as high-band 8 mm tape, consumer digital video tape, and computer data recording tape such as AIT and Mammoth.
In obliquely evaporated tape, it is known that the magnetic particles form a plurality of columnar clusters, that is, columns, and that these columns are arranged on the non-magnetic base film. The longitudinal axes of the columns are slightly tilted from the direction perpendicular to the surface of the magnetic layer. Generally, such a structure of a magnetic layer is called a xe2x80x9ccolumnar structurexe2x80x9d. Up to now, as a columnar structure, one wherein the magnetic particles and non-magnetic particles randomly aggregate to form columns has been known. In columns where the magnetic particles and non-magnetic particles randomly aggregate, the diameter of the magnetic particles is generally about 10 nm.
For example, when forming a Coxe2x80x94O thin film as the magnetic layer, the magnetic particles are Co particles (Co crystallites) while the non-magnetic particles are CoO particles (CoO crystallites) containing oxygen at a high content. CoO is known as an antiferromagnetic material having a Nxc3xa9el temperature of about 300 K. The Nxc3xa9el temperature is extremely close to room temperature, so the magnetic anisotropy of Co is not influenced significantly.
The magnetic particles and non-magnetic particles are can be distinguished by high resolution observation using electron beam diffraction or a transmission type electron microscope. It is also possible to analyze the microstructure formed by these particles by combining the above methods and methods of elemental mapping. Here, as methods of elemental mapping, energy dispersive X-ray (EDX) microanalysis and energy-filtered electron microscope analysis can be mentioned (see Digests of the 24th Annual Conference on Magnetics in Japan (2000) (12pA-14, p. 22), and the Journal of The Japan Institute of Metals, vol. 65 (5), 2001 xe2x80x9cMicrostructural Analysis of Obliquely Evaporated Coxe2x80x94CoO Tape Using TEM and EELSxe2x80x9d.)
Summarizing the problem to be solved by the invention, to achieve high density recording, a higher output of reproduction and low noise medium are required. To satisfy these requirements in an obliquely evaporated tape, it is necessary to suitably control the diameter and shape of the magnetic particles in the magnetic recording medium and the columnar shape.
The electromagnetic conversion characteristic S/N is improved by making the size of the magnetic particle smallers and by reducing variation in the particle size. However, no method has yet been found for forming a magnetic layer having small size magnetic particles and less variation in particle size. A magnetic recording medium controlled in size and variation of size of the magnetic particles forming the columnar structure has therefore been desired.
An object of the present invention is to provide a method of producing a magnetic recording medium able to control the size and variation in size of magnetic particles forming the columnar structure.
Another object of the present invention is to provide a magnetic recording medium having a magnetic layer reduced in the size and variation in size of the columnar structure magnetic particles.
According to a first aspect of the present invention, there is provided a method of producing a magnetic recording medium comprising the step of forming a cobalt-oxygen (Coxe2x80x94O) magnetic layer on a non-magnetic base film running at a constant speed by continuous oblique evaporation depositing metal vapor of volatilized cobalt incident obliquely to a surface of the non-magnetic base film, wherein said evaporation is conducted at a film forming rate, defined as an average deposition rate of the magnetic layer at a part of the non-magnetic base film exposed to the incident metal vapor, of not less than 0.5 xcexcm/s to form an internal microstructure of the magnetic layer comprising a plurality of columns each having a diameter of not more than about 15 nm constituted by magnetic particles of Co crystallites having a size of not more than about 10 nm connected in chains in a direction substantially perpendicular to the magnetic layer and non-magnetic particles of CoO crystallites packed between the columns and separating the columns from each other.
Preferably, crystals of the magnetic particles are oriented in a longitudinal direction of the columns and the size of the magnetic particles is a substantially minimum size within a range where the magnetic particles do not exhibit super paramagnetism.
Preferably, a plurality of the magnetic layer are stacked.
Preferably, the method further comprises the step of forming a protective film of the magnetic layer on the magnetic layer.
Preferably, an incident angle of the metal vapor to the surface of the non-magnetic base film is restricted by using one or more masks provided with an opening through which the metal vapor passes.
More preferably, the minimum incident angle of the metal vapor to the surface of the non-magnetic base film is 45xc2x0 to 60xc2x0.
According to a second aspect of the present invention there is provided a method of method of producing a magnetic recording medium comprising the step of forming a magnetic layer on a non-magnetic base film running at a constant speed by continuous oblique evaporation depositing metal vapor of volatilized metal magnetic material incident obliquely to a surface of the non-magnetic base film, wherein said evaporation is conducted at a film forming rate, defined as an average deposition rate of the magnetic layer at a part of the non-magnetic base film exposed to the incident metal vapor, of not less than a predetermined rate to form an internal microstructure of the magnetic layer comprising a plurality of columns each having a diameter of not more than about 15 nm constituted by magnetic particles having a size of not more than about 10 nm connected in chains in a direction substantially perpendicular to the magnetic layer and non-magnetic particles packed between the columns and separating the columns from each other.
Preferably, crystals of the magnetic particles are oriented in a longitudinal direction of the columns and the size of the magnetic particles is a substantially minimum size within a range where the magnetic particles do not exhibit super paramagnetism.
Preferably, the step of forming the magnetic layer comprises the step of making the magnetic layer a partially oxidized film of the metal magnetic material by supplying oxygen during deposition of the metal vapor on the non-magnetic base film.
According to a third aspect of the present invention there is provided a magnetic recording medium comprising a non-magnetic base film and a cobalt-oxygen (Coxe2x80x94O) magnetic layer formed on the non-magnetic base film by evaporation, said magnetic layer having an internal microstructure comprising a plurality of columns each having a diameter of not more than about 15 nm constituted by magnetic particles of Co crystallites having a size of not more than about 10 nm connected in chains in a direction substantially perpendicular to the magnetic layer and non-magnetic particles of CoO crystallites packed between the columns and separating the columns from each other.
According to a fourth aspect of the present invention there is provided a magnetic recording medium comprising a non-magnetic base film and a magnetic layer formed on the non-magnetic base film by evaporation, said magnetic layer having an internal microstructure comprising a plurality of columns each having a diameter of not more than about 15 nm constituted by magnetic particles having a size of not more than about 10 nm connected in chains in a direction substantially perpendicular to the magnetic layer and non-magnetic particles packed between the columns and separating the columns from each other.